diff --git a/src/renderer/Window.cpp b/src/renderer/Window.cpp
index ad608f8..3dff060 100644
--- a/src/renderer/Window.cpp
+++ b/src/renderer/Window.cpp
@@ -381,6 +381,15 @@ void Window::OnResized()
       return;
    SDL_GetWindowSize(m_nwnd, &m_width, &m_height);
    SDL_GetWindowSizeInPixels(m_nwnd, &m_pixelWidth, &m_pixelHeight);
+   // vpinball-wasm: keep the (GL) default back buffer's tracked size in sync
+   // with the window's actual pixel size, mirroring what the BGFX backend
+   // already does explicitly every frame (RenderDevice.cpp's swapchain
+   // resize handling). Without this, glViewport(0, 0, m_width, m_height) in
+   // RenderTarget::Activate() keeps using the pre-resize size forever, so
+   // e.g. entering browser fullscreen (which resizes the canvas well after
+   // the back buffer was created) only renders into the old, smaller area.
+   if (m_backBuffer)
+      m_backBuffer->SetSize(m_pixelWidth, m_pixelHeight);
 }
 
 VPX::Window::VideoMode Window::SDLtoVPXVideoMode(const SDL_DisplayMode* mode)